While the new findings shouldn't spur parents to start mega-dosing their kids with the vitamin, most children could benefit from a little more sunshine, Dr. Michal L. Melamed of the Albert Einstein College of Medicine in the Bronx, New York, told Reuters Health. And giving children a multivitamin containing 400 international units (IUs) of vitamin D is a good idea too, she added, especially in the winter months.

A few small studies have suggested that vitamin D deficiency may be widespread among US children, Melamed and her team note in their report. To get a sense of how common the problem is nationwide, they looked at data on 6,275 children and young adults one to 21 years old from the 2001-2004 National Health and Nutritional Examination Survey.

Nine percent were classified as deficient in vitamin D, meaning the concentration of the nutrient in their blood was below 15 nanograms per milliliter (ng/mL), while 61% were vitamin D insufficient, with levels between 15 and 29 ng/mL.

Deficiency was more common in older children, girls, obese individuals, those who drank milk less than once a week and those who spent more than four hours a day in front of a TV, computer or video screen. Non-Hispanic black children and Mexican-Americans also were more likely than whites to be deficient in vitamin D.

The researchers also found that kids with vitamin D deficiency were more likely to have high blood pressure, low calcium levels (important for bone growth), and low levels of good HDL cholesterol than children with adequate amounts of the vitamin.

If vitamin D deficiency does turn out to increase heart disease risk, as the cholesterol and blood pressure figures suggest, this could have long-term consequences for the health of Americans, Melamed said.

Parents can help their children get enough vitamin D by making sure they spend at least 15 to 20 minutes in the sun without sunscreen, Melamed said. But the ability of the skin to produce vitamin D from sun-as well as the risk of skin cancer--varies depending on skin color, she added. Dark-skinned children may need an hour or more outdoors without sunscreen, while it might be best for very fair kids with a family history of melanoma not to spend any time outdoors without sun protection, according to the researcher.

There are definitely worries about getting too much sun, and what we're advocating is not sunbathing, Melamed said.

And vitamin D supplements must be used cautiously as well, she added. Taking too much can lead to kidney stones and other kidney problems, Melamed said. Taking more than 400 IU a day, which is the current recommendation, may not be safe.

But it's OK to take 400 IU a day, the amount commonly contained in multivitamins, and especially important to do so in the wintertime, when people get less sun and vitamin D levels drop, she added. In Melamed's study, just 1 in 25 of the study participants had taken 400 IU of vitamin D daily for the past month.

Other research has shown vitamin D deficiency among US teens climbed sharply between 1988 and 1994, Melamed noted. Definitely in the last 20 years there's been an increase in vitamin D deficiency.

One factor is likely more scrupulous use of sunscreen, she said. There's also been less milk drinking, less outdoor activities, more indoor activities, watching TV and playing computer games, she added. All of these things have kind of contributed to what we're seeing.